/Engine/Source/Runtime/Core/Public/Math/Quat.h
assumes normalized quaternion and twist axis
void ToSwingTwist ( const [TVector](API\Runtime\Core\Math\TVector)< T > & InTwistAxis, [TQuat](API\Runtime\Core\Math\TQuat)< T > & OutSwing, [TQuat](API\Runtime\Core\Math\TQuat)< T > & OutTwist ) const